The present invention relates generally to extruder screws and, in particular to an extruder screw with a mixing section.
Extruder screws are used for transforming solid materials into a molten mixture for subsequent use in a mold or die via extrusion or injection. Materials that are used for this purpose include a range of plastic materials, as well as various metal or thixotropic materials. The screw consists of a generally cylindrical body with at least one helical thread machined thereon. The outer diameter of the threads is known as the peak of the screw, the areas between the peaks are known as channels of the screw. The opposing ends of the screw are known as the feed end, where the solid material is introduced, and the discharge end, where the molten material is delivered.
Extrusion, injection molding or blow molding with a single screw extruder, includes feeding the solid material in pellet, chip, powder, or flake form to the feed end of the extruder through a hopper or similar device mounted on an opening of a heated cylindrical barrel. The extruder screw is rotatably mounted and received in the barrel. The screw thread maintains a minimum clearance to the barrel and the material is moved downstream through the channel from the feed end to the discharge end by forces exerted by the rotation of the screw. The solid material fed into the screw channel is compacted into a solid plug or solid bed and the solid bed melts as it travels down the screw channel.
Extruder screws are often divided into a plurality of sections along the length of the screw from a feed section adjacent the feed end of the screw, where the solid material is introduced to the screw, to a metering section adjacent the discharge end of the screw, which delivers the molten material in a constant amount for extrusion or injection. Often, the feed section and the metering section each have a constant barrel clearance. The section or sections between the feed section and the metering section of the screws can include, but is not limited to, a taper section, an energy transfer section, and a mixing section. It is in these intermediate sections where the majority of the melting of the solid material into the molten material occurs.
Because the majority of the melting of the solid material occurs in the mixing section, it is desirable to maximize the amount of mixing and efficiency of the melting in the mixing section and thereby increase the overall efficiency of the extrusion, injection molding, or blow molding process. It is also desirable to provide an extruder screw mixing section that may be constructed with a minimal amount of machining.